1. Field of the Invention
The present invention relates to a control device of a work positioning apparatus, and more specifically to a control device of a work positioning apparatus for adjusting a work position to a given position in a three-dimensional space at the time of welding with a welding robot.
2. Description of the Related Art
Conventional work positioning apparatuses (positioners) for adjusting a work position at the time of welding with an arc welding robot include a rotating shaft, an inclination shaft for rotating a work fixed onto a stage rotatable with the rotating shaft in a vertical plane, and a vertical shaft for vertically moving the stage. In the case of welding a work that largely extends off a stage with such a positioner having three shafts, if a posture of the work is changed, the work would collide with the floor or the positioner. To overcome this problem, a method of preventing a work from interfering with the floor or other such obstacles (for example, see Japanese Unexamined Patent Application Publication No. 2005-161376).
The method as disclosed in Japanese Unexamined Patent Application Publication No. 2005-161376 calculates a degree of interference with the floor or the like at the time of rotating, inclining, or vertically moving a work based on an external size of the work and the degree at which the work decenters from a rotation center of the stage in a to-and-fro direction, a side-to-side direction, and a vertical direction of the work, and sends a signal right before the work interferes with the floor or the like to bring the positioner to an emergency stop in response to the signal.
However, the method as disclosed in Japanese Unexamined Patent Application Publication No. 2005-161376 sends a signal right before a work interferes with the floor or the like and thus involves a possibility of the work colliding against the floor or the like. This is because a stage being moved cannot suddenly stop. In short, it takes a certain period of time to completely stop rotations of motors for driving three shafts of a positioner after the motors receive signals for stopping each motor, and a stage having the work fixed thereonto is coasting during that period. In particular, a heavier or longer work moves at higher acceleration and thus, coasts for a longer time and has a higher possibility of colliding with the floor or the like.